The present invention relates to a field emission cathode having a multiplicity of emitting tips. The invention moreover relates to a method for the fabrication of a field emission cathode and to the application of a field emission cathode.
Field emission cathodes have by their very nature certain advantages over thermal cathodes. Field emission cathodes develop less heat than thermal cathodes, a fact which is of considerable advantage particularly in instruments which are operated at low temperatures or which are intended to operate in a vacuum generated by a cryogenic pump. Moreover, field emission cathodes can be baked out more easily, as a rule, and the carbon fiber cathodes described herein are less sensistive to poor vacuum conditions than thermal cathodes.
It is known from the prior art to use a single graphite fiber as a field emission cathode (J.Phys.D: Appl. Phys., Vol. 7, 1974, pages 2105-2115). However, a field emission cathode consisting of a single graphite fiber is unable to furnish high emission currents. In addition, it is unduly sensitive and relatively unstable.
Moreover, it has been known for a long time already to use thin, pointed wires made from metals with high melting points, such as tungsten and molybdenum, as field emission cathodes. Field emission cathodes are known, for example from J.Appl.Phys. 41, 1970, page 7681, having a multiplicity of emitting tips in order to generate a higher emission current. A multiple tip cathode known according to prior art contains forty tungsten wires which form a spot-welded, comb-like structure. Another field emission cathode discussed in this publication primarily comprises a multiplicity of fine parallel tungsten needles and is produced by oriented cooling of a nickel-tungsten alloy, etching off of the nickel matrix for exposure of the parallel tungsten needles formed during solidification, and electrolytical shaping of the needles to afford pointed tips. The reciprocal distance of the tips is relatively large on the average compared with the thickness of the tungsten needles forming the tips, and that way relatively high field emission currents can be generated, of the order of one milliampere. However, these tungsten needle cathodes are extremely sensitive to overloads and ion impact, which at relatively high vacuums may lead to permanent damage and changes of the emission capacity.